Conventionally, a structure in which a core comprising a wind frame portion provided with winding, and an upper and lower flange portions formed to extend at opposite ends of the wind frame portion is fixed by bonding to a pedestal made of synthetic resin mounted with external connection terminals. Further, end portions of the winding are tied to and soldered to roots of the external connection terminals as a main stream of the surface-mount coil such as the choke coil.
Recently, however, a structure wherein lead frames having external connection terminals are directly fixed through an adhesive or the like to the bottom face of the lower flange portion of the core made of ferrite, ceramic, or the like, for example, has been frequently employed in order to decrease the height. Such external connection terminals are separated from the core, and the base of the core is bonded only to the core mount surfaces of the lead frames.
FIG. 6 is a perspective view of a conventional surface-mount coil seen from a bottom face side (mount board side) and which is of a type as disclosed in Japanese Patent Application Laid-open No. 9-213539, wherein an electrode is directly fixed by bonding to the lower flange portion of the drum-shaped core.
In FIG. 6, a surface-mount coil 20 comprises a coil body including a core 5, and a wind frame portion 3 of which winding (not shown) is applied. The core 5 having the wind frame portion 3 and upper and lower flange portions 1 and 2 is formed to extend at opposite ends of the wind frame portion 3. Lead frames 10 and 11, include external connection terminals 6 and 7 to be surfaces of which solderable layers (such as tinning layers, copper plating layers, gold plating layers, solder plating layers, and the like which have a solder wetting property equal to or better than that of lead frame material) are bonded. Core placing portions 8 and 9 extend from the external connection terminals and have widths larger than widths W of the external connection terminals. A bottom face of the lower flange portion 2 of the coil body is fixed by bonding with an adhesive or the like to upper faces 8a and 9a (lower invisible faces in FIG. 6) of the core placing portions 8 and 9 of the lead frames, and end portions of the winding are conductively connected by soldering to packing terminals 15 and 16 of lead frames 10 and 11.
Height differences H are formed at boundary portions between the external connection terminals 6 and 7 and the core placing portions 8 and 9. The external connection terminals are separated from the core, and the base of the core is bonded only to the core mount surfaces of the lead frames. Reference numerals 13 and 14 designate engaging projections formed on the lead frames 10 and 11 for positioning lead frames 10 and 11 on the coil body.
In the above-described conventional surface-mount coil 20, by disposing the wide core placing portions 8 and 9 higher (by height differences H) than external connection terminals 6 and 7 on the lead frames 10 and 11 (by a bending process), insulation between a wiring pattern in the mount board face right under the lead frames and the lead frames can be maintained.